Depth perception is a life-or-death ability for lambs inhabiting rugged mountain terrain. How quickly a lamb develops that faculty may depend on the amount of time it spends with its ewe. Thirteen sets of lamb littermates were the subjects of an experiment that addressed that question (107). One member of each litter was left with its mother; the other was removed immediately after birth. Once every hour, the lambs were placed on a simulated cliff, part of which included a platform of glass. If a lamb placed its feet on the glass, it “failed” the test, since that would have been equivalent to walking off the cliff. Below are the trial numbers when the lambs first learned not to walk on the glass—that is, when they first developed depth perception.

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Number of Trials to Learn Depth Perception Group Mothered, x, Unmothered, yi 12 2 3 4 5 6 7 8 9 10 11 12 13 2 SWN 3 5 3 2 1 1 5 3 I 735 3 [I 10 5544 4 8 12 7